A U.S.-Russian brokered ceasefire deal for southwestern Syria took effect at noon (0900 GMT) on Sunday, the latest international attempt at peacemaking in the six-year war.

The United States, Russia and Jordan reached a ceasefire and "de-escalation agreement" this week with the aim of paving the way for a broader, more robust truce.

The announcement came after a meeting between U.S. President Donald Trump and Russian President Vladimir Putin at the G20 summit of major economies in Germany.

Several ceasefires have crumbled since the onset of the conflict and it was not clear how much the combatants - Syrian government forces and the main rebels in the southwest - were committed to this latest effort.

With the help of Russian air power and Iranian-backed militias, Syrian President Bashar al-Assad's government has put rebels on the back foot over the last year. The wide array of mostly Sunni rebels include jihadist factions and other groups supported by Turkey, the United States and Gulf monarchies.

Earlier talks between the U.S. and Russia about a "de-escalation zone" in southwest Syria covered Deraa province on the border with Jordan and Quneitra, which borders the Israeli-occupied Golan Heights.

A senior State Department official involved in the talks said further discussions would be necessary to decide crucial aspects of the agreement, including who will monitor its enforcement.

Russian Foreign Minister Sergei Lavrov said the deal includes "securing humanitarian access and setting up contacts between the opposition in the region and a monitoring center that is being established in Jordan's capital."

The multi-sided Syrian conflict, which grew out of popular protests against Assad's rule in 2011, has killed hundreds of thousands of people and created the world's worst refugee crisis.